1. Field of the Invention
The present invention generally relates to non-reciprocal circuit elements, and, more specifically, to a non-reciprocal circuit element, such as an isolator and a circulator, for use in the microwave band.
2. Description of the Related Art
Generally, non-reciprocal circuit elements, such as isolators and circulators, have a characteristic that permits a signal to be transmitted only in a predetermined direction but not in the opposite direction. By utilizing this characteristic, for example, isolators are used in transmitting circuits of mobile communication devices, such as automobile phones and cellular phones.
A two-port isolator shown in, for example, FIG. 6 of Japanese Unexamined Patent Application Publication No. 2003-046307 is known as a non-reciprocal circuit element of the type described above. As shown in FIG. 6 of the above-cited document, in the two-port isolator, first and second center electrodes are disposed on a surface of a ferrite so that the first and second center electrodes intersect each other while being insulated from one another. A resistor is connected between one end of the first center electrode that is connected to an input port and one end of the second center electrode that is connected to an output port. An inductor is connected in series with the resistor.
This two-port isolator realizes an insertion loss bandwidth and an isolation bandwidth that are tolerable for practical use by setting the intersection angle between the first and second center electrodes to about 40 to 80 degrees. The inductor is arranged to compensate a phase shift resulting from a difference of the intersection angle from 90 degrees. However, in the two-port isolator, widening of the insertion loss bandwidth undesirably narrows the isolation bandwidth. Conversely, widening of the isolation bandwidth undesirably narrows the insertion loss bandwidth.
In addition, a two-port isolator shown in FIGS. 6 and 7 of International Publication No. WO2007/046229 is also known. In the two-port isolator, first and second center electrodes are arranged on a ferrite so that the first and second center electrodes intersect each other with being insulated from one another. One end of the first center electrode is connected to an input port, whereas the other end of the first center electrode and one end of the second center electrode are connected to an output port. The other end of the second center electrode is connected to a ground port. Furthermore, a matching capacitor and a resistor are connected in parallel between the input port and the output port.
This two-port isolator advantageously reduces an insertion loss significantly. However, widening of the isolation bandwidth is desired for this two-port isolator.